In combine harvesters equipped with one or more axially arranged threshing rotors, straw walkers are mounted downstream of the threshing rotors to transport crop material towards the rear of the combine whilst grain and chaff that falls through the straw walkers is recuperated. The straw walkers are usually shaped as transport frames with a saw-tooth shaped profile, arranged side by side, and subjected to a reciprocating motion which drives the transport of the crops. A phase difference between adjacent straw walkers is applied in these reciprocating movements, usually by mounting the straw walkers on a pair of crank shafts with suitably designed crank portions.
Straw walkers are generally mounted at a forwardly inclined angle, designed in conjunction with the shape and angle of the saw-tooth shaped stages, so as to obtain an efficient transport of the crops when harvesting on essentially flat terrain, in average weather conditions and for a number of crop types. When harvesting uphill or downhill however, transport and discharge of the crops may deteriorate. On any type of terrain, flat or otherwise, optimal operation of the straw walkers may be difficult when harvesting in humid conditions or when certain crop types are harvested which require specific transport forces.
One solution that has been proposed is to make the speed of the reciprocating motion adjustable, as illustrated for example in WO-A-2014/106633, where the speed of the straw walker motion is automatically adjusted to a measurement of the combine's inclination on the field. This improves the efficiency and adaptability of the straw walker operation in changing conditions, but it is still open to improvement in terms of optimizing said operation. EP-A-1817951 proposes a system wherein the straw walkers are not supported by crank shafts but by a plurality of variable length actuators. This solution allows to adapt the shape of the path that defines the reciprocating motion of the straw walkers. It is a solution that allows for a very flexible operation of the straw walkers, and in that regard it does answer to the above-named technical problems. A drawback of this system however is that it is technically very complex, requiring pneumatic or electric actuators, subjected during prolonged periods of time to substantive reciprocating forces. The system is therefore likely to be expensive and/or vulnerable to malfunctioning. DE-A-19649020 describes a combine wherein the straw walkers are mounted on eccentrically rotating bearings coupled to an axle with a fixed diameter instead of to a crank shaft. The degree of eccentricity may be adjustable. This too brings flexibility to the straw walker operation, but it is also quite a complex and expensive solution.